Scattered radiation is substantially generated by the interaction between primary radiation, emanating from the focus of a radiation source, and the object to be examined. Scattered radiation impinging on a radiation convertor of a radiation detector from a different spatial direction than the primary radiation as a result of this interaction causes image artifacts in the reconstructed image.
Thus, collimators are placed upstream of the radiation convertors in order to reduce the detected proportion of scattered radiation in the detector signals. By way of example, known collimators comprise absorber elements, which are arranged next to one another in a collimation direction and are aligned in a unidirectional fashion in respect of their longitudinal extent. In the radial direction, the absorber surfaces of the absorber elements are aligned in a fan-shaped fashion with respect to the focus of a radiation source, and so only radiation from the spatial direction in the direction of the focus can impinge on the radiation detector. By contrast, scattered radiation proportions are absorbed by the absorber surfaces of the absorber elements.
A slight tilt of the absorber elements compared to an intended alignment, or erroneous positioning of the absorber elements, or the entire collimator, compared to the radiation convertor, already leads to shadowing of the active regions of the radiation convertor and hence to falsification of or reduction in an attainable signal-to-noise ratio. Hence, a particular challenge when assembling a radiation detector is, firstly, to manufacture the collimator in a very precise shape and, secondly, to align the collimator very precisely with respect to the radiation convertor. Here, positional accuracy of the order of a few 10 μm must be attainable and also verifiable by metrological means.
The integration of the collimator into the radiation detector is complicated by the fact that the active regions of the detector elements in the radiation convertor are, for the most part, no longer visible from the outside when aligning the collimator. In indirect-conversion radiation convertors, in which the radiation is converted indirectly into electrical signals via the generation of light pulses by an incident X-ray quantum, the scintillator array used to generate the light pulses is covered by an opaque cover-reflector on the side of the beam incidence direction. Hence the structuring of the scintillator array is no longer visible from the outside during the integration of the collimator.
JP 2003 177 181 AA and U.S. Pat. No. 6,982,423 B2 have disclosed radiation detectors, in which the collimator is produced in small units and, in the form of tiles or a matrix, is screwed to a radiation detector or radiation detector module. Moreover, embodiments are known in which the collimator modules are directly adhesively bonded onto the radiation convertor. Alternatively, the collimator modules in this case are aligned with respect to the outer edges of the radiation convertor. However, in the known cases, possible faulty positioning or tilting of the absorber elements of the collimator can only be detected in a subsequent test when the radiation detector has been completely assembled. Replacing a collimator module in such a collimator is very costly and requires much time.